I. Field of the Invention
The present invention relates to the field of droplet ejectors. More specifically, the present invention relates to droplet ejectors whose excitation is locally controlled, as is the case in ink-jet printers.
II. Description of the Related Art
Many types of droplet ejectors exist, with substantial prior art describing and supporting them. Some droplet ejectors work by ejecting a continuous stream of fluid and subsequently re-directing part of the jet to a specific location. Other types of ejectors, typically classified as drop-on-demand ejectors, produce a drop only when they receive a signal to eject the drop. The invention herein described is of the drop-on-demand type.
Fundamentally, an ejector will release a droplet when the kinetic energy at the liquid-nozzle-ambient interface exceeds the surface tension and adhesion energy of the interface. Several methods are used in order to impart sufficient kinetic energy to the fluid. In certain devices, such as spray nozzles and fuel injectors, pressure is applied to the bulk fluid with a pump. In drop-on-demand devices the energy is often provided thermally or acoustically. Focused acoustic energy, as described in U.S. Pat. No. 5,591,490 and U.S. Pat. No. 5,111,220, is known to eject droplets, though this approach requires the scanning of the focused beam behind the liquid-ambient interface in order to select the location of droplet ejection. Thermal inkjet printers, however, rely on an array of resistors heating an array of fluid cavities. When a given resistor receives a voltage signal, it will heat the ink such that a bubble will form. The formation of this bubble generates sufficient pressure in the fluid to eject a drop from the nozzle. An advantage of thermal technology is the ease with which droplets can be ejected selectively from an array of cavities.
Acoustic inkjet printers are also known which. rely on a piezoelectric element converting an electrical signal to a mechanical displacement that constricts a fluid cavity. The piezoelectric element essentially acts as a piston, which squeezes out a drop from the nozzle. Recent advances in the art have enabled piezoelectric arrays to selectively eject droplets from an array of nozzles. In both thermal and piezoelectric ejectors, droplet ejection rates are currently limited to approximately 10 kHz.
A disadvantage of thermal ejectors is that the liquid is essentially boiled, which requires specific formulations of ink, for example, and precludes the ejection of volatile or organic compounds sensitive to heat.
Piezoelectric ejectors appear to overcome many of the thermal ejectors"" limitations, but have some drawbacks of their own. In order to generate sufficient pressures for droplet ejection, substantial displacement is required of the piezoelectric, which limits its ejection rate. Furthermore, fabricating arrays of piezoelectric elements capable of providing relatively large displacements at higher frequencies is a difficult and costly process.
The ejection of droplets by squeezing a fluid cavity with electrostatic force is also generally known for certain applications. These applications are, however, limited, and the fluid is typically subjected to large electric fields, which can charge the liquid or damage the constituents of a solution or suspension of interest. Although methods to reverse the effects of charging have been attempted, such as disclosed in U.S. Pat. No. 5,818,473, damage to the solutions can still occur, for example to sensitive biochemical solutions.
What is needed, therefore, is a droplet ejector capable of ejecting droplets at rates faster than 10 kHz which will neither heat the liquid nor subject it to damaging electric fields. Furthermore, the ejector should be small enough and individually addressable such that an array of ejectors can deposit patterns of droplets quickly, as in printing.
It has been recognized by the present inventors that a judiciously designed cavity with a nozzle and filling channel can be acoustically excited at its resonance frequency and that such resonance will increase the pressure at the nozzle such that droplet ejection occurs. The displacement required of the exciting element is small enough to allow the excitation to be generated by a conventional piezoelectric element or a vibrating diaphragm. It has further been recognized by the present inventors that the resonant cavities can be small enough, and the excitation frequencies high enough to enable addressable arrays of ejectors to generate droplets at a rapid rate and in patterns.
It is an object of the present invention to provide an ultrasonic droplet ejector from an ultrasonic excitation source, a resonant cavity, and a nozzle.
It is an object of the present invention to provide an ultrasonic droplet ejector with a nozzle and a filling channel such that the flow resistance of the nozzle is sufficiently below that of the filling channel to ensure ejection from the nozzle rather than regurgitation back into the filling channel.
It is an object of the present invention to provide a resonant cavity ultrasonic droplet ejector whose excitation source is a piezoelectric element.
It is an object of the present invention to provide a resonant cavity ultrasonic droplet ejector whose excitation source is a vibrating diaphragm whose motion is generated by the electrostatic attraction of the diaphragm towards a second electrode such that the liquid of interest is not subjected to an electric field.
It is an object of the present invention to provide a resonant cavity ultrasonic droplet ejector where each droplet ejection requires more than one cycle of acoustic excitation, but where the droplet ejection rate is higher than 10 kHz.
It is a further object of the present invention to provide an array of resonant ultrasonic droplet ejectors where each ejector in the array can be independently excited.
The present invention achieves the above objects, among others, by providing a method of forming resonant cavities where at least one wall of the cavity contains an ultrasonic excitation source, where one wall of the cavity contains a nozzle, and where the cavity is connected to a refill channel.